This invention relates to subtractive developers for lithographic plates and more particularly to a subtractive developer for lithographic plates of the wipe-on type as well as of the presensitized type, utilizing a coating of diazo resin on a substrate.
The purpose of a subtractive developer is to adequately desensitize the background area while, at the same time, leaving the image area unaffected. The developer should also provide a coating of gum to protect the background area.
If the developer does not desensitize completely the unexposed diazo resin in the background area, the background area, during use of the developed plate in a printing press or during storage of the plate, may be accidentally sensitized resulting in smudging of the background. Further requirements for an effective subtractive developer for negative working lithographic plates are that it be relatively fast in action such that the plate may be developed preferably in a few seconds, that it require no additional gumming of the plates after development, that it be relatively stable during storage such as to be provided with a reasonably practical shelf life, and that it not be subject to producing noxious or disagreeable fumes and odors during use.
A further requirement for a lithographic plate subtractive developer, due to the now quite prevalent use of automatic developing machines, is that the developer be free of noxious and corrosive effects upon the metallic parts of the developing machine and more particularly upon the copper plumbing generally in use in such apparatus.
All those requirements are fulfilled by the developer of the present invention which may be safely used in automatic lithographic plate developing machines, which is relatively fast acting, which does not release any obnoxious or undesirable fumes and odors, which is effective in appropriately desensitizing and gumming the background areas of a lithographic plate without affecting the image areas, and which is endowed with a long shelf life.